Precompression effect in pump body

ABSTRACT

The current application discloses various embodiments where a pre-compressive force is applied to a defined zone of the fluid end of a pump so as to extend the operational life of the fluid end by reducing stress and fatigue level at the defined zone of the fluid end. In one embodiment, the defined zone comprises one or more recesses near the piston bore of the fluid end. In another embodiment, the defined zone comprises one or more recesses near the inlet bore of the fluid end.

RELATED APPLICATION DATA

This application claims priority of U.S. Provisional Patent Application Ser. No. 61/308657 filed Feb. 26, 2010, which is incorporated by reference herein.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. All references discussed herein, including patent and non-patent literatures, are incorporated by reference into the current application.

The invention is related in general to wellsite surface equipment such as fracturing pumps and the like. Hydraulic fracturing of downhole formations is a critical activity for well stimulation and/or well servicing operations. Typically this is done by pumping fluid downhole at relatively high pressures so as to fracture the rocks. Oil can then migrate to the wellbore through these fractures and significantly enhance well productivity.

Multiplex reciprocating pumps are generally used to pump high pressure fracturing fluids downhole. Typically, the pumps that are used for this purpose have plunger sizes varying from about 9.5 cm (3.75 in.) to about 16.5 cm (6.5 in.) in diameter. These pumps typically have two sections: (a) a power end, the motor assembly that drives the pump plungers (the driveline and transmission are parts of the power end); and (b) a fluid end, the pump container that holds and discharges pressurized fluid.

In triplex pumps, the fluid end has three fluid cylinders. For the purpose of this document, the middle of these three cylinders is referred to as the central cylinder, and the remaining two cylinders are referred to as side cylinders. Similarly, a quintuplex pump has five fluid cylinders, including a middle cylinder and four side cylinders. A fluid end may comprise a single block having cylinders bored therein, known in the art as a monoblock fluid end.

The pumping cycle of the fluid end typically is composed of two stages: (a) a suction cycle: During this part of the cycle a piston moves outward in a packing bore, thereby lowering the fluid pressure in the fluid end. As the fluid pressure becomes lower than the pressure of the fluid in a suction pipe (typically 2-3 times the atmospheric pressure, approximately 0.28 MPa (40 psi)), the suction valve opens and the fluid end is filled with pumping fluid; and (b) a discharge cycle: During this cycle, the plunger moves forward in the packing bore, thereby progressively increasing the fluid pressure in the pump and closing the suction valve. At a fluid pressure slightly higher than the line pressure (which can range from as low as 13.8 MPa (2 Ksi) to as high as 145 MPa (21 Ksi)) the discharge valve opens, and the high pressure fluid flows through the discharge pipe.

Given a pumping frequency of 2 Hz, i.e., 2 pressure cycles per second, the fluid end body can experience a very large number of stress cycles within a relatively short operational lifespan. These stress cycles may induce fatigue failure of the fluid end. Fatigue involves a failure process where small cracks initiate at the free surface of a component under cyclic stress. The cracks may grow at a rate defined by the cyclic stress and the material properties until they are large enough to warrant failure of the component. Since fatigue cracks generally initiate at the surface, a strategy to counter such failure mechanism is to pre-load the surface.

Typically, this is done through an autofrettage process, which involves a mechanical pre-treatment of the fluid end in order to induce residual stresses at the internal free surfaces, i.e., the surfaces that are exposed to the fracturing fluid, also known as the fluid end cylinders. US 2008/000065 is an example of an autofrettage process for pretreating the fluid end cylinders of a multiplex pump. During autofrettage, the fluid end cylinders are exposed to high hydrostatic pressures. The pressure during autofrettage causes plastic yielding of the inner surfaces of the cylinder walls. Since the stress level decays across the wall thickness, the deformation of the outer surfaces of the walls is still elastic. When the hydrostatic pressure is removed, the outer surfaces of the walls tend to revert to their original configuration. However, the plastically deformed inner surfaces of the same walls constrain this deformation. As a result, the inner surfaces of the walls of the cylinders inherit a residual compressive stress. The effectiveness of the autofrettage process depends on the extent of the residual stress on the inner walls and their magnitude.

Co-pending and co-assigned PCT application PCT/IB2010/053867, which was filed on Aug. 28, 2010 and claiming the priority of U.S. Provisional Application Ser. No. 61/239639 filed on Sep. 3, 2009, discloses a pump body that is pre-compressed by expanding a displacement plug in a cavity to pre-compress a portion of a pump body so as to reduce the fatigue level of the pump body during operation.

It remains desirable to provide improvements in wellsite surface equipment in efficiency, flexibility, reliability, and maintainability.

SUMMARY

The present invention in one embodiment applies pre-compressive forces in pump bodies, or selected portion(s) thereof, to inhibit initiation of fatigue cracks in the fluid end of a multiplex pump.

In one aspect of the current application, a fluid end of a pump is provided, where the fluid end comprises a piston bore, an inlet bore, an outlet bore and at least one pre-compressive element that creates a pre-compressive force in a defined zone of the fluid end. In some cases, the defined zone is one or more recesses near the piston bore of the pump body. In some other cases, the defined zone of the pump body is one or more recesses near the inlet bore of the pump body. In one embodiment, the pre-compressive element is a displacement plug mounted on the pump body. In another embodiment, the pre-compressive element is a raised surface on the fluid end.

According to another aspect of the current application, there is provided a method of reducing fatigues of a fluid end of a pump, said method comprising, providing a fluid end comprising a piston bore, an inlet bore, and an outlet bore; providing a pre-compressive element; and using the pre-compressive eminent to create a pre-compressive force in a defined zone of the fluid end. In some cases, the defined zone is one or more recesses near the piston bore of the pump body. In some other cases, the defined zone of the pump body is one or more recesses near the inlet bore of the pump body. In one embodiment, the pre-compressive element is a displacement plug mounted on the pump body. In another embodiment, the pre-compressive element is a raised surface on the fluid end.

According to a further aspect of the current application, there is provided an assembly comprising a plurality of pump bodies each defining a piston bore, an inlet bore, an outlet bore and at least one raised surface element on one of the pump bodies thereof; at least a pair of end plates disposed on an outside portion of the pump bodies; and a plurality of fasteners connecting the pump bodies and end plates to form the pump assembly, the raised surface engaging on an adjacent pump body or an adjacent end plate, the fasteners and raised surface element providing a pre-compressive force in a defined zone of the pump body. In some cases, the raised surface element is on the surface exterior of the pump body.

According to one embodiment, the raised surface element has a uniform thickness. According to another embodiment, the raised surface element does not have a uniform thickness. In some cases, the raised surface element is a part of the pump body. In some other cases, the raised surface element is an independent and additional part of the pump body. The raised surface element can be made of a material different from the material of the pump body.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of the fluid end of a triplex pump assembly according to an embodiment of the application.

FIG. 2 is an exploded view of the triplex pump assembly of FIG. 1 according to an embodiment of the application.

FIG. 3 is a perspective view of one of the pump body of the triplex pump assembly of FIGS. 1-2 according to an embodiment of the application.

FIG. 4 is a side sectional view of the pump body of FIGS. 3 as seen along the lines 4-4 according to an embodiment of the application.

DETAILED DESCRIPTION OF EMBODIMENTS OF APPLICATION

FIGS. 1-2 show the fluid end of the multiplex pump 100 including a plurality of pump bodies 102 secured between end plates 104 by means of fasteners, which in one case comprise one or more tie rods 106 and one or more threaded nuts 156. The end plates 104 are utilized in conjunction with the fasteners 106 to assemble the pump bodies 102 to form the pump 100. When the pump 100 is assembled, the three pump bodies 102 are assembled together using, for example, four large fasteners or tie rods 106 and the end plates 104 on opposing ends of the pump bodies 102. At least one of the tie rods 106 may extend through the pump bodies 102, while the other of the tie rods 106 may be external of the pump bodies 102. In addition to the triplex configuration of pump 100, those skilled in the art will appreciate that the pump bodies 102 may also be arranged in other configurations, such as a quintuplex pump assembly comprising five pump bodies 102, or the like.

As best seen in FIGS. 3-4, the pump body 102 has an internal passage or piston bore 108 which may be a through bore for receiving a pump plunger through the fluid end connection block 109. The connection block 109 provides a flange that may extend from the pump body 102 for guiding and attaching a power end to the pistons in the pump 100 and ultimately to a prime mover, such as a diesel engine or the like, as will be appreciated by those skilled in the art.

The pump body 102 may further define an inlet port 110 opposite an outlet port 112 substantially perpendicular to the piston bore 108, forming a crossbore. The bores 108, 110, and 112 of the pump body 102 may define substantially similar internal geometry as prior art monoblock fluid ends to provide similar volumetric performance. Those skilled in the art will appreciate that the pump body 100 may comprise bores formed in other configurations such as a T-shape, Y-shape, in-line, or other configurations.

In one embodiment, a raised surface 150 extends from an exterior surface 152 of the pump body 102, best seen in FIG. 3. The raised surface 150 may extend a predetermined distance from the exterior surface 152 and may define a predetermined area on the exterior surface 152. While illustrated as circular in shape from a uniform thickness in FIG. 3, the raised surface 150 may be formed in any suitable shape.

In one embodiment, the end plates 104 may further comprise a raised surface 154, best seen in FIG. 2, similar to the surface 150 on the pump body 102 for engaging with the raised surfaces 150 of the pump body 102 during assembly.

As the tie rods 106 are torqued (via nuts or the like) to assemble the pump assembly 100, the raised surfaces 150 on the pump body 102 and raised surfaces 154 on the end plates 104 engage with one another to provide a pre-compressive force to the areas 114 of the pump body 102 adjacent the intersection of the bores 108, 110, and 112. The pre-compressive force is believed to counteract the potential deformation of the areas 114 due to the operational pressure encountered by the bores 108, 110, and 112. By counteracting the potential deformation due to operational pressure, stress on the areas 114 of the pump body 102 is reduced, thereby increasing the overall life of the pump bodies 102 by reducing the likelihood of fatigue failures. Those skilled in the art will appreciate that the torque of the fasteners 106, 156 and the raised surfaces 150, 154 cooperate to provide the pre-compressive force on the areas 114.

In one embodiment, the raised surface 150, 154 is a disk of diameter D and of thickness t centered substantially on point 121. In some cases, the raised surface 150 is identical for each pump body 102 and is present on each exterior surface 152 of the pump body 102. Each raised surface 150 of each pump body 102 will match the raised surface of another pump body portion or the raised surface 154 of an end plate 104. By knowing the material type used for the pump body 102, i.e. its physical properties, the diameter D and the thickness t, it is possible to optimize the pre-compressive force applied through the torque of the tie rods 106. Alternatively, by defining the torque that will be applied by the tie rods 106, it is possible to optimize the pre-compressive force applied through the diameter D and the thickness t.

In one embodiment, the raised surface 150 may be made from the same material as the pump body 102 and forms a monolithic portion of the pump body 102. In another embodiment, the raised surface 150 may be an independent part from the pump body 102. In another embodiment, the raised surface may be made from a different material from the pump body 102. In another embodiment, the raised surface may be present at only one side of exterior surface 152 of the pump body 102. In another embodiment, the raised surface 150 is not centered on point 121.

According to another embodiment, the raised surface 150 is not identical for each pump body portion and is not necessarily present on each exterior surface 152 of the pump body 102. Each raised surface of each pump body portion will interact with the raised surface of another pump body or from the raised surface 154 of an end plate 104. As well, the raised surface 154 of an end plate 104 can be identical or different from a raised surface 150 from a pump body portion. By modeling interaction of each raised surface from one pump body with another pump body or end plate, it is possible to optimize the pre-compressive force applied through the torque of the tie rods 106, through the geometry of the raised surface.

According to one embodiment, the stress created by the raised surface 150, 154 is a perpendicular or substantially perpendicular force to the exterior surface 152. According to another embodiment, the stress can be applied on a different axis, or on a different plan. According to a further embodiment, the stress can be applied differently through the raised surface. If the thickness of the raised surface is not uniform, for example the raised surface is slightly larger at bottom (from piston bore 104) and slightly smaller at top (from piston bore 108), it is possible to apply pre-compressions having different values at the bottom or at the top of the raised surface. In this example, the pre-compression will be higher at the bottom than at the top of the fluid end pump body.

According to a further aspect, in one embodiment, hydraulic jacks are used to provide pre-compression on fluid end blocks via hydraulic tensioners. In this way, it is possible to modify the pre-compression value with the hydraulic tensioners based on pump load. The pre-compressive force applied through the torque of the tie rods 106 can therefore be optimized based on pump load. As discussed above, other parameters such as material properties, geometry of the raised surface can be taken into account to optimize the desired pre-compressive force.

According to one embodiment, the geometry of the raised surface is such that the pre-compressive force is applied to the areas 114 of the pump body 102 adjacent the intersection of the bores 108, 110, and 112. Geometry can be optimized to counteract the potential deformation of the areas 114 due to the operational pressure encountered by the bores 108, 110, and 112. When the raised surface is a disk, by increasing the diameter D of the raised surface, the pre-compressive force is applied to a larger region 114. By counteracting the potential deformation due to operational pressure, stress on the areas 114 of the pump body 102 is reduced, thereby increasing the overall life of the pump bodies 102 by reducing the likelihood of fatigue failures.

According to a further aspect, in one embodiment pre-compressive force is applied to the areas 130 of the pump body portions 100, which represents one or more recesses near the bore 108. According to a second embodiment pre-compressive force is applied to the areas 140 of the pump body 102, which represent one or more recesses near the bore 110. The pre-compressive force for area 130 can be applied, for example with a displacement plug 131 which is mounted and forced on the pump body portion. Similarly, the pre-compressive force can be applied on the area 140 with a displacement plug (not shown) which is mounted and forced on the pump body portion.

In one embodiment, the displacement plug 131 is placed in, for example, a drilled bore or cavity formed in the body 102 and expanded with the use of an expansion tool and/or application of a radial force to the drilled bore or cavity, as will be appreciated by those skilled in the art. The bore formed in the body 102 may be cylindrical for a cylindrical plug 116, or tapered to accommodate a tapered plug 116 therein. Other variations are also possible. For example, the pre-compressive force in an embodiment may also be hydraulically or pneumatically applied pressure, for example, via suitable sealed hydraulic or pneumatic connections to the cavity. The pre-compressive force in an embodiment may be applied by injecting a liquid or semi-liquid material into the bore that expands as it solidifies, the expansion of the material providing the pre-compressive force. In another embodiment where the plug 131 is permanently expanded or otherwise larger than the cavity in which it is received in the pump body 102, the plug 131 displaces the area around the plug, maintaining stresses against the abutting surface of the cavity.

Due to the substantially identical profiles of the plurality of pump body 102, the pump body 102 may be advantageously interchanged between the middle and side portions of the assembly 100, providing advantages in assembly, disassembly, and maintenance, as will be appreciated by those skilled in the art. In operation, if one of the pump bodies 102 of the assembly 100 fails, only the failed one of the pump bodies 102 need be replaced, reducing the potential overall downtime of a pump assembly 100 and its associated monetary impact. The pump body 102 is smaller than a typical monoblock fluid end having a single body with a plurality of cylinder bores machined therein and therefore provides greater ease of manufacturability due to the reduced size of forging, castings, etc.

While illustrated as comprising three of the pump bodies 102, the pump 100 may be formed in different configurations, such as by separating or segmenting each of the pump bodies 102 further, by segmenting each of the pump bodies 102 in equal halves along an axis that is substantially perpendicular to the surfaces 152, or by any suitable segmentation.

The preceding description has been presented with reference to some illustrative embodiments of the Inventors' concept. Persons skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structures and methods of operation can be practiced without meaningfully departing from the principle, and scope of this invention. Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope. 

1. A fluid end of a pump, said fluid end comprising: a piston bore, an inlet bore, an outlet bore and at least one pre-compressive element that creates a pre-compressive force in a defined zone of the fluid end.
 2. The fluid end of claim 1, wherein the defined zone is one or more recesses near the piston bore of the pump body.
 3. The fluid end of claim 1, wherein the defined zone of the pump body is one or more recesses near the inlet bore of the pump body.
 4. The fluid end of claim 1, wherein the pre-compressive element is a displacement plug mounted on the pump body.
 5. The fluid end of claim 1, wherein the pre-compressive element is a raised surface on the fluid end.
 6. A method of reducing fatigues of a fluid end of a pump, said method comprising: providing a fluid end comprising a piston bore, an inlet bore, and an outlet bore; providing a pre-compressive element; using the pre-compressive eminent to create a pre-compressive force in a defined zone of the fluid end.
 7. The method of claim 6, wherein the defined zone is one or more recesses near the piston bore of the pump body.
 8. The method of claim 6, wherein the defined zone of the pump body is one or more recesses near the inlet bore of the pump body.
 9. The method of claim 6, wherein the pre-compressive element is a displacement plug mounted on the pump body.
 10. The method of claim 6, wherein the pre-compressive element is a raised surface on the fluid end.
 11. An assembly comprising: a plurality of pump bodies each defining a piston bore, an inlet bore, an outlet bore and at least one raised surface element on one of the pump bodies thereof; at least a pair of end plates disposed on an outside portion of the pump bodies; and a plurality of fasteners connecting the pump bodies and end plates to form the pump assembly, the raised surface engaging on an adjacent pump body or an adjacent end plate, the fasteners and raised surface element providing a pre-compressive force in a defined zone of the pump body.
 12. The assembly of claim 11, wherein the raised surface element is on the surface exterior of the pump body.
 13. The assembly of claim 11, wherein the raised surface element has a uniform thickness.
 14. The assembly of claim 11, wherein the raised surface element does not have a uniform thickness.
 15. The assembly of claim 11, wherein the raised surface element is a part of the pump body.
 16. The assembly of claim 11, wherein the raised surface element is an independent and additional part of the pump body.
 17. The assembly of claim 11, wherein the raised surface element is made of a material different from the material of the pump body.
 18. The assembly of claim 11, wherein the defined zone of the pump body is areas of the pump body adjacent the intersection of the piston bore, the inlet bore, and the outlet bore.
 19. The assembly of claim 11, wherein at least another raised surface element is present on one of the pump bodies thereof and the raised surfaces interacts together to create the pre-compressive force in the defined zone of the pump body.
 20. The assembly of claim 11 wherein the pre-compressive force extends the operational life of the assembly by reducing stress at the intersection. 